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Friday, 7 June 2019

Defiant Radio Set

CWS Defiant A55 Germanium Transistor Radio Set
My mother and father purchased a radio set, about the time they were married (around 1960).
I remember it from my childhood as it was always on in the mornings.
The radio is a Defiant brand set made by the Cooperative Wholesale Society. There is quite a lot about this type of radio on the internet, so I won't go into the history here. Mine is an A55 model. 

It belongs to an era when radios had germanium transistors. With hind-sight we know now that was quite short period, only about a decade, from the end of the fifties to the mid sixties. Valved radio sets were regarded as old fashioned, and the silicon transistor had not yet become commonplace.
I have had the radio up in the loft for many years, and last time I tried it, the audio was quite distorted.

Small modern capacitors wired in parallel with the old.
I thought it would be a good idea to replace the electrolytic capacitors. They are the yellow components in the picture at left. They have a red end and a black end to indicate their polarity. 

These components deteriorate with age, because they have a liquid electolyte, The seals can fail and the capacitors dry out. This type of capacitor is often used between sections of the circuit, where they allow the signals to pass through (these are a.c. currents at audio frequencies) whilst preventing direct current from flowing, thus keeping each transistor correctly biased. 

Modern components are very much smaller than the 1960 types, so I decided to leave the old capacitors in place on the board, cutting one wire so they were no longer in circuit. Then I soldered replacement parts on the underside of the board, thus keeping the appearance of the PCB the same, but hopefully restoring full circuit function. 

After replacing all six capacitors, I switched on the set, which initially worked well - but next time I turned it on it didn't. Sometimes it worked, sometimes the audio was weak and distorted. The Defiant has an unusual power-supply arrangement. It was powered from a PP11 type battery, which is fact two 4.5V batteries in the same box. This provided a split rail supply. The circuit runs from 9 Volts, but the speaker is connected to the mid-point of the batteries. This was done to avoid the need for a large electrolytic capacitor in series with the loudspeaker, and ensured a good bass response on the audio. But it also means that the On/Off switch has to be a double-pole type, in order to switch both positive and negative rails. The on/off switch on my Defiant was intermittent on one, or both sides.

On/Off switch seen here in the "off" position, 
 On/Off switch seen here in the "off" position,
On/Off switch seen here in the "on" position, 
And here in the "on" position. 
Rather than replace the On/Off volume control with a modern part, I took the switch off the back of the potentiometer. This was quite easy to do and just required bending some metal tabs. The switch had some old dried grease in it. I used some spray switch cleaner to soften and remove a lot of the grease, but hopefully leave the internals clean and lubricated. The switch works really well now.

I am happy to report that the set now sounds fantastic, on both long and medium wavebands. BBC radio 4 is still broadcasting on longwave in the UK. 198kHz.

The other things I have done are cleaning – the case was very dirty.  I also glued a bit of scrim back over the hole in the back of the cabinet.

The set has 6 transistors, OC45’s in the RF section and an OC75 as an audio driver, with two OC74’s in the output stage. There were also 6 electrolytic capacitors, ranging from 8uF to 250uF.

I have taken the spun brass disks out of the control knobs and polished out the tarnish – then gave them a coat of varnish to prevent them discolouring again.

the disks were quite easy to remove by poking the end of a cocktail stick through the little hole in the plastic moulding. The glue was hard and dry, and I replaced it with a little dab of hot-melt glue.

Flaky paint on the dial
Unfortunately the paint on the rear of the dial is flaking off, which is a great shame and difficult to fix. I don’t think there’s much that can be done there.

Since PP11 batteries are no longer available I am using a 6-pack of AA cells, I connected a wire to the mid point of the battery pack to provide the split-rail supply that I mentioned earlier.

The set has a particularly rich and full sound. Here are a few more photos.

Hugh M0WYE

Thursday, 23 May 2019

Sound of a Reed Warbler Singing

Recorded the sound of this reed warbler singing in the ditch by the road one day, thought I would share it on here - the bird itself is completely hidden from view, very secretive!
Video of Reed Warbler singing
Hugh M0WYE

Thursday, 8 November 2018

Citroen Berlingo Heater Blower Resistor Change

Now the autumn mornings are getting foggier and colder, a good windscreen demister is essential. My 2004 Citroen Berlingo heater blower was working on fan speeds 3 and 4, but the slow speeds were dead. The cause was a burned-out dropper resistor.

The way it works is this: There are three, wire-wound resistors, to obtain the fastest speed (speed four) the fan motor gets the full supply voltage, with no dropper resistor. A single resistor is switched in series with the fan in position three. Position two has two resistors, and for the slowest speed all three resistors are in circuit. The resistor block is in the air-flow to keep it cool - in fact it is in the same plastic housing as the blower motor itself.

I obtained a replacement resistor block from Ebay, and measured all the resistances. I reproduce my hand-written notes about it below, for those that are interested.

The Haynes manual always cautions to remove the battery negative terminal before working on the electrics, but in this case the circuit only becomes live when you have the engine running. Set the fan switch to off, just to be sure.

In order to change the part you need to know where it is. On this car it is behind the glove-box on the passenger side. If you could easily remove the glove-box, the job would be very easy, but I can't see a way to do that. There is a soft panel underneath which is held in place by some horrible push-fit fasteners. They need to be prised out with a flat-blade screwdriver. There is a small piece of plastic trim on the side of the "transmission tunnel" which needs to be removed. It clips in place and is held by a single Torx screw.

Pushing the passenger seat right back, kneeling on the ground alongside the car, you can reach up behind the fascia to release the resistor from the blower. It has a connector with four wires.

In fact there is a little gap at the left-hand side of the glove-box where you can peek in and see the resistor. A small torch will help greatly.

If you push on the lever on the white plastic bit (closest to you in the photo) you can slide the white bit up slightly, and withdraw it from the blower. There is just enough wire to pull the resistor down below the fascia so that you can see to release the black connector. This is released by two little levers either side of the wires, you squeeze them together and pull the connector out. Reassembly (as they say) is a straight reversal of the above process.

But look carefully - there is a difference in the profile of the two white plastic housings. I had to modify the new one on the right to look like the old one on the left, with a pair of side-cutters and a small file. Otherwise, it just won't fit!

The picture at left shows the housing which has been modified to fit.

Don't be tempted to run the fan without the resistor in place - without the cooling airflow the resistor will get very hot . There is a thermal fuse which will go permanently open circuit if the resistor gets too hot, as would be the case if the motor in the blower failed. This is a safety device to prevent fire and should not be bypassed.

I am not sure why the new part had a different profile. It seems to work perfectly well, with four useful fan speeds and no nasty burning smells. I have messaged the Ebay seller to ask, and will post here any further information. The seller's description matched the model and year of my vehicle so, unless the seller made a mistake, it should have been the correct part.

Hugh M0WYE

Thursday, 19 July 2018

BAT42 Diode Reverse Characteristics

Here's an odd thing. A graph from a Vishay datasheet for Schottky diode type BAT42. Clearly very temperature dependent, but the curious thing is what happens at zero volts - the current keeps flowing! So, for example we can see that at 25 degrees C, when there is 0 Volts across the diode, there is nearly 0.1uA flowing. Perhaps there is a rapid drop to zero hidden by the frame of the graph, or have we discovered a new source of energy?

Still, I thought I had better make sure, so checked with a meter. The ambient temperature, this evening, happens to be 25 degrees C, but just to make doubly sure I also tried applying some heat from a hairdryer. frustratingly, no matter how hot the diode got, the meter remained stubbornly on 0.0 uA. No free power then.


Monday, 28 May 2018

Entwistle PBXN Pickups - Precision Bass Pickup Replacement

Continuing my work on the Hohner Arbor Series P-Bass guitar. Replacing the Hohner single-coil pickups with a humbucking pair. But what kind of pickup to fit? Frankly the instrument isn't worth spending a lot of money on, so I was attracted to the Entwistle range of pickups.

The designer, Alan Entwistle, has an impressive biography and has clearly been in the business of designing guitars for years. I liked the idea of using neodymium magnets, too, because, in theory, more magnetism = more output. And I am all for improving the signal to noise ratio.

So I arranged to buy some from Wrexham Guitar Shop - Jan was very helpful and even called me "Dude" which was cool.
When the pickups arrived, they were just wrapped up in a bit of bubble wrap in a padded bag. The first thing I noticed was that a couple of the wires were hanging by a thread - and one of them fell off almost immediately. To make matters worse, the wires are soldered to small brass ferrules in the plastic bobbin of the pickup. When you heat the ferrule up to make a soldered joint the plastic melts, so you have to be very quick. On the other side of the ferrule is a hair-thin copper wire. It is all very unsatisfactory.

The photo at left shows the repair I had make when one of the ferrules melted right out of the plastic. Not pretty. You can see the bobbin with the cover removed (it is glued on with a hard, brittle glue).

Eventually I got all the wires resoldered and checked the resistance and inductance to make sure we had a good connection, and then I fitted the new pickups into the instrument. Fortunately, I had already traced out the circuit, but if you are stuck there are some good help sheets on the Entwistle site:  (though no instructions are included with the pickups). I took the opportunity to replace the potentiometers with brand new parts, as these had become crackly.

Now, before I took out the old pickups, I measured the output. In order to get a repeatable result I plucked the string at the 13th fret, holding it down to the fret-board with the tip of the plectum and letting it go. This always gave the same amount of displacement to the string, so it should ring with the same amplitude each time. I used the peak-hold function on the meter and took an average of ten "twangs". I was getting an output of about 80mV.

Imagine my disappointment after fitting the new pickups to measure only 5mV. I hooked the guitar up to an amplifier. There was some sound there, but it was weak, and I had to turn the volume up so that it was horribly noisy. Not to put too fine a point on it, as supplied, these pickups are crap.

Why? There seemed to be plenty of magnetism, put them two close together on the bench and they snap together strongly. The resistance and inductance measurements were good, indicating that there were lots of turns of wire and that the coils were electrically connected. It was a bit of a mystery.

After a bit of investigation with my hall effect magnetic probe, I worked out that the pickups had been assembled wrong. There are two bar-magnets under each pickup, and these should have the same pole facing each other. This "forces" the magnetic field up the pole-pieces to where the strings are. The pickups I had had the magnets arranged North-south, so the field simply went across the bottom of the pickup. The problem was very clear if you touched the pole pieces with a small screwdriver, the field was so weak that the tip of the screwdriver would not stick to the pole-piece.

But what to do... was there any chance of getting one of the magnets off and rotating it? Having already made a bit of a mess with the soldering on one of the pickups, I didn't fancy my chances of returning them, so I decided to have a go. The glue that is used is very hard and it is not easy to pry the magnets off. I used a sharp screwdriver and a blunt knife, and worked a little groove in the plastic underneath the magnet, so some leverage could be applied. It is quite risky applying a lot of force with sharp tools, and I was lucky to get away with nothing more than a grazed knuckle. You have been warned!

Having removed the magnet, I then became aware of another design flaw in these pickups. The pole pieces are threaded and have a screwdriver slot, so you can adjust them - right? Wrong! - there is so much hard glue around the thread of the pole-pieces there is no way they are ever going to move. Don't even try - you will just chew up the screwdriver slot and spoil the appearance of the pickup.

Fortunately the pickups are mounted on springs so you can adjust the height of the pickup under the strings by using the mounting screws to raise or lower one end of the pickup.
The photo above shows the springs. It also shows the blocks of foam which were under the original pickups. The Entwistle pickups have the magnets occupying some of this space, but I thought it was a good idea to put some foam underneath - apart from anything, it stops the wires from rattling.
So with the magnets prized off and refitted with some hot-glue ... was it worth the effort?

Well, yes, we have lots of output - over 100mV. The field cancellation is good too, measured in the Helmholtz coils. But best of all, playing the guitar in the church with the hearing loop - not a trace of feedback or sound from the microphones in the bass amp - so that's a result.

Monday, 21 May 2018

Measuring magnets with a hall effect probe

Permanent magnets vary a lot in strength. The position of the North and South poles can be in unexpected places too. At school we compared magnets by how many paperclips they could lift, and it is possible to plot the field lines using iron filings and a compass needle. But these are rather inexact methods.   It would be nice if there was a digital magnet-meter.
It turns out that such a thing is quite easy to make. I have found one very useful for investigating the magnets in guitar pickups.

I used a hall-effect device from a company called Melexis. I bought it from RS components, in a pack of five, and they worked out at just under a pound each. The particular component I used seems to be obsolete, but hall effect devices are still being used, so I'm sure it would be possible to find something
suitable. I plug the probe into a digital voltmeter to get a reading.

The data sheet is here:

The device runs off a 5V supply, so I used a 78L05 regulator. It might have been better to have used something a little more accurate, because the output voltage swings from just above 0 Volts, maximum north-south, through 2.5V with no field to 5V maximum south-north, and that mid-point of 2.5V end up being a few mV offset if the supply isn't accurate. Anyway, the circuit is given in the datasheet. I used this:

The circuit runs off a 9 Volt PP3 type battery and connects to a Digital Voltmeter. I built it on a small piece of veroboard. The veroboard is actually covered with thin transparent tape. You need to be able to see where the sensor is and you need to be able to get it very close to the surface of the magnet you are measuring. So avoid having components that are taller than the device close to it, or they will get in the way. But the tape is needed because some magnets are conductive and cause short-circuits. The photos show it with the tape removed.

The diagram at left shows how it is laid out, using chip capacitors, and breaking the veroboard track between pins 1 and 2.

You need some fairly fine soldering. It helps to make sure the copper strips are really clean and shiny, use a very fine tipped soldering iron and some thin solder.

Calibration is quite straightforward, because the sensitivity is given in the data sheet, you can work out how many volts you get for a given value of milliTesla. If you wanted to calibrate "properly" you will need to put the device in a known magnetic field, perhaps generated in the Helmholtz coil that I describe elsewhere.

I drew a graph in Excel based on the figures in the table above.

Now some magnets are too strong to measure - they read either 0 or 5V depending on which pole, but you can always space the sensor away from the magnet to be able to compare the strength to a different one.
It is worth noting that the device is not sensitive enough to use as an electronic compass. If you amplify the output you will soon find that the limiting factor is noise. The Hall effect device is quite noisy, so magnets like the Earth's magnetic field, which is of the order of 50 micro Tesla, are lost in the noise from the device.
But for comparing the strength of the magnet in one guitar pickup with another, it is ideal.

And here's a picture of my hall effect probe being used in this way.

Hope you found it interesting.
Hugh M0WYE

Monday, 7 May 2018

Building a Helmholtz Coil

I wonder if anyone else who regularly plays an electric guitar in different venues, has the problem I have ... the hearing aid induction loop is picked up by the pickups in the electric guitar and is amplified by the guitar amp. This amplified sound is then picked up by the mikes, amplified and fed into the hearing aid loop, causing a "howl-round" or feedback situation. With the hearing aid loop running right round the building it is very hard to get away from it.

A guitar pickup is a coil of wire with a permanent magnet inside it. The steel strings move in the magnetic field and generate an alternating current in the coil. The trouble is that any alternating magnetic fields will also be picked up by the coil and amplified. The usual problems are caused by hum fields around the mains transformer in the guitar amp, but a lot of halls and churches now have induction loops that provide a signal for people with hearing aids that work on the same principle.

Now the obvious solution is to use a "humbucking" pickup. This type of guitar pickup has two coils, wound in opposite directions, and wired in series, so that any background magnetism is cancelled out. One coil has a magnet running north-south, the other is south-north so the signal from the guitar strings is in-phase and adds together. It is the perfect solution, background hum is cancelled and the wanted signal is doubled.

My bass guitar is a Hohner Arbor Series "Precision" bass.  I blogged about it a few weeks ago. It looks very similar to a Fender Precision Bass, and, since 1957, these instruments have been fitted with a pair of pickups wired in a humbucking configuration. Wikipedia page about the Fender Precision Bass

So what's going on? Why do I have such problems with my bass?
Replacement pickups are available for a few tens of pounds, but if I changed them, how would I know whether the problem was solved? I need some way of exposing the guitar to a magnetic field like the one from the hearing aid loop, in a controlled way, so that some comparisons can be made.

I did some reading about hearing aid loops, and they have to meet certain standards (BS7594 / IEC60118-4). They are designed to produce a field strength of 100mA/m. They have compression circuits to keep the field strength within that range even when the person speaking into the microphone varies their volume. So 100mA/m looks like the field-strength to aim for.

I decided to make a Helmholtz Coil. Wikipedia Page about Helmholtz Coils
This sort of apparatus looks like a pair of hoops. When fed with a current (d.c. or a.c) the coils produce a uniform field in the space between the two coils. It is also possible to do a sum, based on the number of turns of wire, the diameter of the coil and the current flowing trough it, to work out the exact strength of the field inside.
Some sort of former was required to wind the coils on, and I found an old cable drum in the garage which has a diameter of about 40cm. The two coils must be spaced at a distance which is the same as the radius of the coil, in this case it is 19cms. The body of the guitar would fit inside this, with the pickups in the uniform part of the field.

The equation above looks a bit scary, but it turns out that if we want to calculate the field in terms of mA/m we can leave out the permeability, and the 4/5ths raised to the power of 3/2 works out to be 0.71554.
It works out that with 53 turns of wire on each coil, 1mA of current will produce the 100mA/m field strength required. I wired the two coils in series and put a 100 ohm resistor in series with the both of them. Driving the coil from my Levell Oscillator easily achieves 1mA. I put my True RMS Multimeter across the resistor. By ohms law, 1mA produces a voltage of 100mV across the resistor.

But I'm getting ahead of myself, I need to make a former to wind the wire on ...

After separating the end-cheeks of the cable drum (required an angle grinder!) I drew a circle with a radius of 19cm (piece of string and a pencil). I cut some small pieces of wood from a strip of pine and glued them round the circle I had drawn, with PVA glue. Repeated the task on the other half and let the glue set hard.

I found a small sheet of thin MDF, and cut that up to make the inside edge of the former for the wire. Glued that on with PVA and, again, left the glue to set.

Then I wound the 53 turns of wire on the former. To make this easier I found an old plastic pill-pot which was an exact fit in the centre hole of the wooden disk. I could use that as an axle to rotate the disk around. I set up the spool of wire on a big screwdriver in a vice so that it would dispense wire freely. I made a mark on the wooden disk so I could count the turns. I used 0.56mm dia. enamelled copper wire - but almost any type of wire would suffice.

Once wound, the two disks were joined together with wooden spacers. I used proper brass screws for this, because any steel in, or around the coils, will tend to distort the magnetic field.

The photo below shows the final set up with a guitar in the coils. the Levell oscillator is feeding an a.c. signal into the coils and the meter is used first to set the current to 1mA (100mV across the 100 ohm resistor) and then to measure the output from the guitar pickups.

Finally, here's the bass guitar in the coil. I tried it vertically and horizontally, but it didn't make much difference to the measurements, although the pickups are closer to the centre of the coil in the horizontal position shown.

As long as the frequency is below 1kHz, the meter reads very accurately. If I wanted to look at how the pickups performed at higher frequencies, I would need to use an oscilloscope or a different kind of meter.

So what do the results show?
The black-coloured guitar is a useful comparison because it has hum-bucking pickups in bridge and neck positions and a single coil pickup in the middle. I measured the output of the guitars at three different frequencies, 50Hz, 100Hz (mains hum frequencies) and at 800Hz, more representative of a hearing aid loop. On the black guitar the humbucking pickups had almost no signal at any frequency - and the background hum (with the oscillator switched off) was low too. The middle, single coil pickup picked up the Helmholtz signal quite strongly giving an output of about 10mV at 800Hz. But the output from the bass was twice as strong with over 20mV. So clearly the bass pickups are behaving like single coil pickups - not humbuckers.
Now 20mV is meaningless on its own. When you pluck a guitar string, it is quite tricky to get a consistent output, because it depends how hard you pluck it. I developed a "standard" way of plucking a string. Hold the string down on the fret-board at the 12th fret (in the middle) with the point of a plectrum, then slide the plectrum off the string, this gives the same amount of deflection to the string each time you pluck it. Using this method, and using the peak-hold function on the meter, and averaging 10 separate readings I got the following figures:
E string 89.9mV
A string  57.7mV
D string 56.8mV
G string 80.8mV.
This may seem a bit over the top, but I want to be able to compare the outputs of different pickups. You will see that 20mV is about the quarter of the output of the guitar when a string is plucked. A most unacceptable level of background signal.

The reason that the bass guitar picked up more signal from the Helmholtz coils maybe because the pickups are physically bigger. The signal output is proportional to the area of magnetic flux which the coil encloses, so a larger diameter coil will have more magnetic field lines passing through it.

So the next step is to get some new pickups and see how they perform.
Hugh M0WYE